Glucagon (AQA A Level Biology)

• If the concentration of glucose in the blood decreases below a certain level, cells may not have enough glucose for respiration and may not be able to function normally
• The control of blood glucose concentration is a key part of homeostasis
• Blood glucose concentration is controlled by two hormones secreted by endocrine tissue in the pancreas
• This tissue is made up of groups of cells known as the islets of Langerhans
• The islets of Langerhans contain two cell types:
  • α cells that secrete the hormone glucagon
  • β cells that secrete the hormone insulin

• These α and β cells act as the receptors and initiate the response for controlling blood glucose concentration
• The control of blood glucose concentration by glucagon can be used to demonstrate the principles of cell signalling
  • α and β cells in the pancreas act as the receptors
  • They release the hormone glucagon
  • Liver cells act as the effectors in response to glucagon

The second messenger model

• If a decrease in blood glucose concentration occurs, it is detected by the α and β cells in the pancreas:
  • The α cells respond by secreting glucagon
  • The β cells respond by stopping the secretion of insulin

• The decrease in blood insulin concentration reduces the use of glucose by liver and muscle cells
• Glucagon (the first messenger) binds to receptors in the cell surface membranes of liver cells
• This binding causes a conformational change in the receptor protein that activates a G protein
• This activated G protein activates the enzyme adenylyl cyclase
• Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP)
• cAMP binds to protein kinase A enzymes, activating them
• Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them
• Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
• Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose
  • This process is known as glycogenolysis and it occurs via the second messenger model

• The enzyme cascade described above amplifies the original signal from glucagon and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level

The second messenger model. Glucagon acts as the first messenger and cAMP acts as the second messenger

• Glucagon and insulin are both involved in the negative feedback loop that controls blood glucose concentration
• When blood glucose concentration is low, more glucagon is secreted and less insulin is secreted
• When blood glucose concentration is high, more insulin is secreted and less glucagon is secreted

The corrective mechanisms work together to regulate blood glucose concentration